title: Deep cortical layers are activated directly by thalamus.
abstract: The thalamocortical (TC) projection to layer 4 (L4) is thought to be the main route by which sensory organs communicate with cortex. Sensory information is believed to then propagate through the cortical column along the L4→L2/3→L5/6 pathway. Here, we show that sensory-evoked responses of L5/6 neurons in rats derive instead from direct TC synapses. Many L5/6 neurons exhibited sensory-evoked postsynaptic potentials with the same latencies as L4. Paired in vivo recordings from L5/6 neurons and thalamic neurons revealed substantial convergence of direct TC synapses onto diverse types of infragranular neurons, particularly in L5B. Pharmacological inactivation of L4 had no effect on sensory-evoked synaptic input to L5/6 neurons. L4 is thus not an obligatory distribution hub for cortical activity, and thalamus activates two separate, independent "strata" of cortex in parallel.
authors.affiliation: Department of Neuroscience and Kavli Institute for Brain Science, Columbia University, New York, NY 10032, USA.
authors.forename: Christine M
authors.initials: CM
authors.lastname: Constantinople
authors.forename: Randy M
authors.initials: RM
authors.lastname: Bruno
mesh_terms: D000818:Animals; D005073:Evoked Potentials, Somatosensory; D019579:Neocortex / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D009474:Neurons / Q000187:drug effects / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D013569:Synapses / Q000187:drug effects / Q000502:physiology; D013788:Thalamus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*